The great significance of specific problems for the progress of mathematics in general and the important role that they play in the work of individual researchers is undeniable. As long as a branch of science offers an abundance of problems, it remains alive; a lack of problems foreshadows extinction or the cessation of independent development. Just as every human undertaking strives towards goals, so mathematical research needs problems. The power of the researcher is tempered through the solution of problems; by them he discovers new methods and points of view and opens up a wider and more expansive horizon.